1. Field of the Invention
The present invention relates to an organic light emitting display apparatus and a method of driving the same, and more particularly, to an organic light emitting display apparatus that preserves a threshold voltage of a driving transistor and a method for driving the organic light emitting display apparatus.
2. Description of the Related Art
Generally, organic light emitting display apparatuses, which electrically excite a fluorescent organic compound and emit light, drive a plurality of organic light-emitting cells arranged in a matrix by using an electrical voltage or an electrical current so as to display variable visual images, such as moving images corresponding to a video signals. Such organic light-emitting cells have diode characteristics and thus are referred to as organic light-emitting diodes (OLEDs).
A contemporary organic light emitting diode (OLED) includes an anode, an organic thin film, and a cathode layer. The organic thin film includes a hole injection layer, a hole transport layer, an emission layer, an electron transport layer, and an electron injection layer in order to provide a good balance between electrons and holes and thus improve the luminous efficiency of the OLED.
Methods for driving this organic light emitting diode (OLED) are classified into a passive matrix method, and an active matrix method that uses thin film transistors (TFTs) or MOSFETs. In a passive matrix method, anodes and cathodes are arranged to intersect with each other, and lines are selected so as to drive pixels. On the other hand, in an active matrix method, a TFT is electrically connected to each ITO pixel electrode, and pixels are driven by voltages preserved by the capacitances of capacitors connected to gates of the TFTs. Active matrix methods are divided into a voltage programming method and a current programming method according to the type of signal applied in order to program and preserve a voltage stored by a capacitor.
A contemporary organic light emitting display apparatus includes an organic light emitting diode OLED, two transistors and a capacitor. TFTs are generally used as a switching transistor and a driving transistor.
When a positive or negative voltage is continuously applied to the gate electrode of the driving transistor, the threshold voltage of the driving transistor is shifted due to the phenomenon of voltage stress. Accordingly, when a general 2-transistor 1-capacitor (2T-1C) circuit is used, degradation of the driving transistor may occur, and several hours after the degradation, the threshold voltage of the driving transistor may be shifted, and thus the brightness of the OLED apparatus is altered and becomes different from the initial brightness of the OLED apparatus. Therefore, this shift of the threshold voltage of the driving transistor needs to be prevented.